1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, and more particularly, to a method of forming an isolation film for semiconductor devices.
2. Description of the Related Art
A typical process for forming a trench-type isolation film in an integrated circuit includes using a mask layer pattern to protect an active region of a semiconductor substrate, forming a trench in a field region of the semiconductor substrate, filling the trench with a dielectric film, flattening the dielectric film, and removing the mask layer pattern. The shape of the isolation film and a step between the isolation film and the active region can affect the characteristics of a gate oxide film formed subsequently. For example, a gate oxide film is typically thinner at a side wall of the trench, that is at the transition between the isolation film and the active region. A voltage applied across the gate oxide film induces charge and electric fields that are greatest where the gate oxide film is thinnest. As a result, the insulating characteristics of the thinnest regions of the gate oxide film can be degraded.
Another factor that affects the characteristics of the gate oxide film is an annealing that increases the isolation film's resistance against a wet etching. For example, when the isolation film is annealed at 1050.degree. C., the wet etching resistance of the isolation film increases. However, the annealing can generate defects such as dislocations in the isolation film. The dislocations can degrade the characteristics of the overlying gate oxide film. In contrast, annealing the isolation film at a higher temperature, for example, 1150.degree. C., avoids defects such as dislocations in the isolation film but may partially thin the gate oxide film.